Alkenes

Question 1

What is the general formula for alkenes?

Answer 1

CnH2n (for aliphatic hydrocarbons).

Question 2

What is the structure and bond angle for alkenes?

Answer 2

Planar, bond angle 120 degrees.

Question 3

What are the two parts to a double bond?

Answer 3

The sigma bond and a pi bond.

Question 4

How does a double bond form?

Answer 4

A sigma bond is formed directly between two carbon atoms by the overlap of orbitals. A pi bond is formed above and below the plane of the carbon atoms by sideways overlap of p-orbitals. In each carbon involved in the double bond, 3 of its electrons go into sigma bonds and 1 goes into a pi bond.

Question 5

When an alkene reacts which bond breaks?

Answer 5

The pi bond breaks.

Question 6

Conditions for the hydrogenation of an alkene?

Answer 6

A hydrogen gas and an alkene is passed over a finely divided nickel catalyst. The conditions are 450k and 1000kPa. This produces an saturated alkane.

Question 7

Write the equation for the hydrogenation of ethene?

Answer 7

C2H4 + H2 —-> C2H6

Question 8

Uses of hydrogenation?

Answer 8

Used in margarine manufacture.

Question 9

Conditions for the halogenation of an alkene?

Answer 9

Alkenes react with halogens at room temperature. This produces a di-substituted halogenoalkane.

Question 10

Write the equation for the halogenation of ethene with bromine?

Answer 10

C2H4 + Br2 —-> C2H4Br2

Question 11

Conditions for the direct hydration of an alkene?

Answer 11

Steam and the gaseous alkene are heated to high temperatures (600k) and high pressures (6MPa) in the presence of a phosphoric acid catalyst. This produces an alcohol.

Question 12

Write the equation for the direct hydration of ethene?

Answer 12

C2H4 + H2O —-> C2H5OH

Question 13

Uses of direct hydration?

Answer 13

Used in ethanol manufacture.

Question 14

Conditions for the electrophilic addition of an alkene?

Answer 14

Electrophiles are attracted to the pi electrons. This occurs at room temperature. A halogenoalkane is formed.

Question 15

Write the equation for the addition of hydrogen bromide to ethene?

Answer 15

C2H4 + HBr —-> C2H5Br

Question 16

What is the mechanism of the addition of hydrogen bromide to ethene?

Answer 16

Hydrogen bromide is a polar molecule.
(H+ - Br-). The electron pair in the pi bond is attracted to the slightly positive hydrogen atom, causing the double bond to break. A bond forms between one of the carbon atoms and the hydrogen atom. The H-Br bond breaks by heterolytic fission and Br- is formed and a carbocation is formed. The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product, bromoethane.

Question 17

Which way do the curly arrows point?

Answer 17

They point in the direction for which the electrons are moving.

Question 18

Addition of bromine to an alkene?

Answer 18

When the bromine approaches the alkene, the electrons in the pi bond repel the electrons in the Br-Br bond. The Br-Br is now slightly polar. The electron pair in the pi bond is attracted to the slightly positive bromine atom, which causes the double bond to break. The Br-Br bond breaks by heterolytic fission and a Br- and carbocation is formed. The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product.

Question 19

What to do with polymers?

Answer 19

Recycling - separate out into types.
Combustion - for energy production.
Cracking - Use as a feedstock.